March 24, 1916.

THE COLLIERY GUARDIAN.

569

THE ROTARY IN MINE WORK/

By C. M. Means.

Direct current is eminently suited for the operation of
certain appliances used in the production of coal, while
alternating current is the better medium for power trans-
mission. Up to now, haulage locomotives arranged for
alternating current have made little progress, and a
successful mining locomotive of this type is not yet in
sight. Mining machines are being built with alternating
current motors, but they have a limited application,
because of the inherent characteristics of the induction
motor.

In small mines, where no haulage js contemplated, it
may be advisable to use mining machines equipped with
induction motors, but where haulage is already installed,
or its future use is possible, it is advisable to consider
only machines operated with direct-current motors.
Other equipment, such as pumps and hoists, can be run
successfully with alternating current applied at the trans-
mission voltage; or by the use of transformers such
voltage can be secured as may be most desirable.

The potential of direct current is limited to 650 volts,
and in the interest of safety it is often desirable to make
the limit 300 volts. For transmission purposes under-
ground, alternating current at much higher pressures
may be used, the exact voltage being dependent upon
the length of the distribution lines and the amount of
power to be transmitted.

In order to get'proper results in a large mine, it is
necessary to transmit the power in the form of alter-
nating current, converting it to direct current for the
operation of locomotives, mining machines, small pumps,
etc. To accomplish this conversion, there is a choice
between the rotary converter and the motor generator
set. In recent years the latter has been so improved
that it is now a reliable piece of apparatus, and has been
applied extensively in many districts where alternating
current is available. Decent developments in the rotary
converter, however, have net only made this device a
competitor of the motor generator set, but present indi-
cations point to its adoption in mining work in preference
to the other machine.

Machines used for converting alternating into direct
current are rated on their ability to carry a constant
load, rather than to take care of the actual operating
cycle of a mine. A mining load is extremely unsteady,
and machinery adapted to carrying wide fluctuations
should be selected for the conditions existing in the
average mine.

In the case of a motor generator set, the capacity of
the machine is limited by the ability of the motor to
keep in step, or that of the; generator to commutate at
overloads. The usual practice is to arrange the motor
so that it will, under proper conditions, carry well up to
100 per cent, overload, but this implies a normal alter-
nating current voltage which cannot always bo main-
tained. Manufacturers customarily design the direct-
current unit to carry 75 to 100 per cent, momentary over-
load without serious flashing at the brushes.

In the case of a rotary converter, the only limiting
feature of short overloads is that of commutation; and
in a properly-designed machine it should be possible to
carry 200 per cent, overload for short periods of time.
Inasmuch as the overloads are of short duration, no
undue heating occurs in the winding of the rotary or
of the transformers used in connection with the outfit.

§

o
£

IGO

60

80

50

70

90

25

200KW

200

125

--TM

150

50	75	100

Per Cen+. Load

600

£400

200

5 PM.

4 PM.

Fig. 1.

The accompanying chart (fig. 1) shows a typical direct-
current mining load. In actual practice it was found
necessary to use two 150 kw. motor generator sets to
handle tliis load, yet it could easily have been carried
by one 200 kw. rotary converter of proper design.

In certain cases it is possible to arrange the work so
as to reduce overloads, but in general an arrangement
that would give any material overload reduction would
so affect the operation of the mine as to result in
decreased output with increased cost. This implies that
the load ordinarily encountered in mine work is a fixed
condition, and equipment must be designed to handle a
demand fluctuating between wide limits, with a low
average value as compared to the maximum. This con-
dition the rotary is eminently fitted to meet.

* Goal Age.

The first cost of a rotary converter, together with
transformers, designed for a certain continuous load,
may be slightly higher than that of a motor generator
set, but the cost of. a rotary to handle the average mining
load should be lower than that of a motor generator set
to perform a like cycle of work. This is because of its
greater overload capacity. The reliability of operation
favours the rotary converter of recent design, as com-
pared to the motor generator set. Transformers are
extremely reliable in every way, and the rotary has a
low-voltage field and armature winding, and is one
machine.

The synchronous motor of a motor generator set
usually has a high-voltage winding and a revolving field
that must be insulated for high induced potentials. The
field and armature winding on the direct-current end,
in the case of 500-volt machines, must be provided with
an exciter. Thus we have two complete machines in
the case of 250 volts, and three in the case of 500 volts.

The efficiency of the rotary, together with its trans-
formers, is high, reference to the curves showing just
what this value is. When the load under which these
machines operate is considered, it is apparent that the
saving in power in a rotary is an important item, and
cannot be ignored. The. actual operating efficiency of a
motor generator set, on a mining load, will be about 70
per cent, or lower, while that of a rotary, with its
transformers, should be 80 per cent, or above.

It is possible to secure considerable over-compounding
on the direct-current end of a motor generator set, while
a rotary will give a flat voltage or a very small amount
of over-compounding. The direct-current voltage of a
motor generator sot is a fixed quantity, and cannot bo
varied beyond the limits fixed by its design. The voltage
of a rotary can be varied by changing transformer taps.
A flat voltage is exactly what is required when a sub-
station is located properly with respect to its work.
Thus, in general, the regulation of the direct-current
voltage of a rotary is better suited to the work than a
voltage that fluctuates with the load.

The rotary converter operates at about unity power
factor, which is ideal so far as this part of- the load is
concerned. If it is desirable to correct the power factor
for other apparatus on the line, a proper condenser
should be used, and so located as to fnrnish the maximum
amount of effective correction, rather than depend on
synchronous motor generator sets, placed without regard
to location for proper correction of power factor, and
operated without consideration of the amount of correc-
tion required at different times.

SEARCH FOR NEW COAL FIELDS.

Dr. Aubrey Strahan, Director of' the Geological Survey of
Great Britain, lectured at the Royal Institution last Friday
evening on “ The Search for New Coal Fields in England.’'’
He said that within the last hundred years there was a saying,

Under red rock no coal,” and it was not until geology had
made considerable progress as a science that it was realised
that some of our most important sources of 'coal lay under the
new red sandstone and other newer formations. The search
for coal was going on all over the country, and to show what
had been done in developing our resources, he instanced the
concealed measures in Yorkshire, Nottinghamshire, and
Denbighshire. With regard to the extension of the Yorkshire
coal field, the lecturer showed by means of diagrams the
principles upon which was based the conclusion that- this coal
field terminates eastwards, in the neighbourhood of the Trent
Valley, thus reducing considerably the limit assumed by the
Royal Commission on Coal Supplies. In the case of the
Denbighshire coal measures, the dip of the beds was such
that they would be rapidly carried below workable limits
under the red rocks. But fortunately a series of parallel faults
lifted them up in steps, and kept them comparatively near the
surface. The Kent coal field differed from all other examples
in being concealed everywhere, and there was nothing from
which its existence could be presumed, except a consideration
of the geology of Northern Europe., In 1911 only 150 tons of
coal were raised; last year the amount was 158,000 tons;
and the total since winning began was 350,000 tons, not a
particularly creditable performance to us as a coal mining
nation. The deepest part of the coal field almost certainly
lay under the Straits of Dover. It had been assumed that the
Kent beds were an extension of the axis of Artois, but it
had to be remembered that the rocks of this axis had been
completely turned head over heels on a stupendous scale, and
there was no evidence of any such movement in Kent. He
did not consider the Kent coal field was on that axis at all,
but it might be compared with a recently discovered coal field
in Belgium, where 12 shafts were being sunk when war broke
out. This coal field lay on the north side of the “ great
wrinkle ” of the earth’s crust, and the measures were fairly
horizontal. Three years ago, he said, he referred to our
happy-go-lucky methods, by which a man could bore how,
and where, and as deep, as he liked without publishing any
record. The Kent borings were for a long while kept secret,
and for a time the only*published account of them appeared in
a German publication, and to-day some of the bores were
described there and nowhere else. The lecture was illustrated
by a number of large drawings and lantern slides showing
the geological structure of the areas described.

Grimsby Coal Exports.—The coal exported from Grimsby
during the week ended March 17 consisted of 1,000 tons to
Dieppe. The total for the corresponding period of last year
was 10,671 tons foreign.

American Pig Iron Production.—The American Iron and
Steel Institute has issued a statistical bulletin showing the
production of pig iron in the United States during 1915.
According to the bulletin, the production by grades was as
follows Basic pig iron, 13,093,214 tons; Bessemer,
10,523,306; foundry, 4,864,348; malleable, 829,821; forge,
316,214; spiegeleisen, 97,885; ferro-manganese, 129,072;
white and mottled, direct castings, ferro-titanium, etc.,
62,253—total, 29,916,213 tons, an increase of 6,583,969 tons
on the previous year. , The largest individual increase
occurred in the case of basic pig iron (increase 3,422,527
tons). The output of pig iron (ah kinds) made with coke and
electricity totalled 29,535,308 tons; anthracite or coke and
anthracite pig iron 84,753 tons ; charcoal and electricity pig
iron, 296,152 tons.

MINING INDUSTRY AND MILITARY SERVICE.

The statement made by Lord Derby in the House of Lords
last week that there were probably in the mines a
great many men who were not really miners, and who had
gone there to escape military service has awakened much
interest. Hitherto, the belief has been general that the
Home Office intended to exempt -all miners from military
service, but Lord Derby has shown that there was a con-
siderable amount of misapprehension on that point. The
authoritative announcement now made is that the Home
Office have agreed that any men who were challenged by
the military authorities -and could not be shown by their
employers before the colliery tribunals to be indispensable
should cease to be exempted.

In Lanarkshire the question of exempting surface labourers
about the mines from military service is causing some trouble
where collieries are double-shifted. Girls can, of course,
be employed on the day shift, but the Mines Act prevents
the employment of female labour at night.

Some of the Lanarkshire employers are anxious that
fortnightly pays should be reverted to in order that the new
clerks may have time to prepare the pay sheets.

At the Nottinghamshire colliery tribunal, the manager of
the New Hucknall Colliery stated that the company had
stuck out against employing men who had been employed
in other trades, but it was found that two had managed to
slip through. Of the 509 employees taken on since the war
began, about 220 were boys under 16. He appealed for all
men as being starred. The military representative pointed
out that no fewer than 63,000 men in Nottinghamshire and
Derbyshire had been starred. He suggested that it was
not justifiable that so many young active men of military
age should be working underground, 'for it would mean that
married men above ground, who could just as ill be spared,
would have to join the Forces. In the case of Sherwood
Colliery, Mr. Heathcote, the manager, said that of the men
now employed at the colliery, 27 could be spared without
seriously affecting the output. Of the New Hucknall
employees the company agreed to relinquish 19 men, and the
others were conditionally exempted, while in the case of
Sherwood 50 were given up, the rest of the claims being
allowed. It was stated that at some collieries there were
now men of 70 years of age working, and quite a number
over 60, who had been in receipt of pensions.

At a sitting of the colliery tribunal for Lanarkshire it
was explained that a number of the pony drivers in Rosehall
Colliery were under 18 years of age, and that the others
might be replaced if men could be got. Lieut. Allan, the
military representative, said the better course would be to
grant exemption to the men. included in the barred classes,
and,he would get the list of names of those who had entered
since August 15. This course was agreed to by the tribunal.
The court then considered the case of men not belonging to
the barred classes. The manager .explained that they had
replaced men by female labour as far as they could, but
these were things that could not be done by female labour.
Lieut. Allan said he did not see why a. number of men who
were only engaged in labouring work could not be replaced
either by lads under military age, or men over military age,
or why the firm could not draw from the large.compensation
list of men fit for light work. He suggested that the men
in question, with the exception of three who were under
18 years of age, ought to be available for military service.
In the result a wagon trimmer was exempted, four pit
labourers, and a boiler fireman were granted exemption for
one month, and a haulage engine man for three months. A
despatch clerk was exempted for three months.

The colliery tribunal in Dean Forest have granted very few
total exemptions to surface workers. As regards screen
workmen, exemption not exceeding two months was the
general rule. Colliery owners were seriously urged to find
substitutes for these men. It Was suggested that women
might act as banksmen; but it was pointed out that the
arduous nature of that class of work rendered it quite out
of the question for this course to be adopted.

Miners whose freedom of choice is hampered by conscien-
tious objections are developing the habit, in Northumberland
and Durham, of disregarding the fact! that the nature of
their employment renders them exempt from military service,
at all events for the present, and are insisting on appearing-
before the local tribunals to state their objection to serving.
Quite a large Humber of such cases have occurred during the
last week or two. At Bedlington, Mr. Hunter, the miners’
agent, pointed out that a miner is only provisionally exempt
so long as the colliery is working, and that therefore miners
who appeal should have their appeals adjourned sine die, so
that they may still appeal if the necessity should arise.

Appealing on behalf of 1,098 of their employees at the
Mansfield Colliery, of the Bolsover Company, Mr. J. P.
Houfton (managing director) stated that this year, up to the
end of February, the shifts lost at the colliery by men absent-
ing themselves numbered 12,686, which was equal to 10 per
cent, of the men employed. The daily output had decreased
by 500 tons. Of the total output, at least 50 per cent, was
used for Government work. The coal exported had decreased
from’40 to 15 per cent. Exemption was granted in all but
87 cases.

At the colliery recruiting tribunal for South Derbyshire,
Mr. R. Hay, manager of the Stanton Colliery, stated that one
man employed was 80 years of age, two were minus their
right hands, and two others had cork feet. None of the
employees at this pit were released for service.—In the case of
a neighbouring colliery, at Boothorpe, it was stated that the
shaft was only 16 yds. deep. The pit employed four men
below and two on the surface. The coal was used for muni-
tions work. All the men were exempted.

At a session at Consett, the secretary of the Consett Iron
Company Limited pointed out that coal drawers, etc., in the
company’s employ were not “ certified,” and that, whilst the
workers at the by-products ovens were badged, those at the
beehive ovens were not.

The military representative at the Weardale rural tribunal
has announced his intention of challenging the whole of the
exemptions granted at a certain session, first on the ground
that the men were not certified and badged, and, second, on
the ground that, at a time of great national crisis, the Wear-
dale Steel. Coal and Coke Company had dispensed with a
number of old men, and replaced them by younger workmen.

An assistant inspector of coal cargoes sought exemption at
South Shields on the ground that his firm was supplying gas
coal for munitions purposes. Elis employers urged, on his
behalf, that the coal must bo inspected both before and after
shipment, and that the examiner must be a man of practical
knowledge. The claim was disallowed.

At Doncaster last week, a firm of mining engineers were
granted exemption for a man who was stated to be engaged